The present invention relates to an improved Gigabaud Link Module for providing a received signal detected indication incorporated with a link unusable signal supplied to a host device.
Gigabaud Link Modules (GLM) are high speed data modules used for transferring massive amounts of data between various pieces of computer equipment, for example between a host computer and a peripheral storage device such as a disc drive. An individual GLM provides bi-directional communication between the parallel format data bus of a host device and a serial format transfer medium. A pair of GLM can be connected to each end of the transfer medium to create a bi-directional communication link, or multiple GLM can be connected in a daisy-chain manner to establish an arbitrated loop configuration. In either case, a GLM module provides bi-directional communication between its host device and the transfer medium, allowing data to be exchanged between the various host devices associated with each of the GLM connected to the transfer medium.
The GLM family specification, FCSI-301 incorporated herein by reference, describes a series of single port serial communications subassembly modules all having a common footprint and configured to support a variety of data rates, transfer media types, transmission lengths, and interface widths. In accordance with the GLM family specification, GLM can be configured to support a variety of transfer media including: short wave laser using 50 or 62.5 micron multimode optical fiber, long wave laser using single mode fiber, and copper using a variety of media and connectors. A first preferred embodiment of the present invention relates to GLM employing laser generated signals and an optical fiber transmission medium in applications supporting Fibre Channel/Arbitrated loop, Local Area Networks, and other shared resource computing arrangements. A second preferred embodiment is also provided having broader application in GLM supporting any of the available transmission media and connected in either a point-to-point or arbitrated loop configuration.
FIG. 1 shows the block diagram of a GLM including a data received signal according to the first embodiment of the present invention. (The block diagrams of the first embodiment and second embodiment shown in FIG. 4 are the same except that the signal detecting circuitry of the second embodiment receives its input from a different point within the receiving circuitry, and the transmitting and receiving circuitry of the second embodiment may be configured to support transfer media other than optical fiber.) The block diagram shown includes function blocks common to all GLM, as well as signal detecting circuitry unique to the present invention. As noted, GLM can be interconnected in two different configurations, either a point-to-point configuration, or an arbitrated loop network configuration. In the point-to-point configuration GLM operate in pairs, with a single GLM connected to each end of a fiber optic link. The output of the transmit portion of a first GLM is connected to one end of an optical fiber, the opposite end of the fiber is then connected to the receive input of a second GLM. The receive input of the first GLM is connected to a second optical fiber, the opposite end of which is connected to the transmit output of the second GLM. Thus, the first GLM transmits data to be received by the second GLM, and receives data transmitted by the second GLM. A central host device can be set up as a hub supporting a number of fiber optic links in a spoke like fashion such that host devices located at the ends of the spokes can communicate with one another through the central hub.
In the arbitrated loop configuration, like the point-to-point configuration, the output of the transmit portion of a first GLM is connected to one end of an optical fiber, the opposite end of which is connected to the receive input of a second GLM. The transmit output of the second GLM, however, is not connected back to the receive input of the first GLM, but rather to the receive input of a third GLM. The transmit output of the third GLM is connected to the receive input of a fourth GLM, and so on in a daisy-chain fashion around a loop until finally, the transmit output of the last GLM in the arbitrated loop is connected to the receive input of the first GLM, thereby closing the loop.
In either configuration, point-to-point or arbitrated loop, the GLM themselves operate in the same manner. A GLM reads data signals from the parallel data bus of its associated host device, and the parallel to serial converter converts the parallel data into a string of serial data bits which are input into a laser driver. The laser driver drives the output of an optical laser diode which converts the electrical signals into optical signals corresponding to the serial data string output from the parallel to serial converter. The laser radiates the signals through the first optical fiber where they are received by a second GLM connected to the opposite end. On the receiving side, a photo diode receives serial optical signals transmitted by another GLM over a second optical fiber. The photo diode converts the optical signals into a serial string of electrical data signals which are then amplified and input into a serial to parallel converter, or deserializer. The deserializer converts the received serial data into a parallel format and writes the parallel data to the parallel data bus of the host device.
In order to synchronize the signals and organize the data being communicated over the serial data link, the host devices connected to each GLM will be equipped with an interface controller. The interface controller organizes the data, and times the exchange of each byte of data between the host and the GLM. An example of such an interface controller configured to support Fibre Channel communications is the TACHYON.TM. Fibre Channel Interface Controller produced by Hewlet-Packard. It should be noted that Fibre Channel is a high speed communication protocol specifically adapted to accurately exchange massive amounts of data quickly and without errors. To ensure successful data exchange between the interface controller and the GLM, a number of status and control signals are exchanged between the GLM and the interface controller along with the parallel data signals. Included among these are the E.sub.-- WRAP and L.sub.-- UNUSE signals.
E.sub.-- WRAP is short for enable wrap. The E.sub.-- WRAP signal is generated by the host's interface controller and enables a data loopback mode in the GLM to check the integrity of the serializer and deserializer circuitry of the GLM. When the interface controller drives this signal true (logic 1) the internal loopback mode is initiated within the GLM. The transmitting laser is shut down, and the interface controller sends test data to the GLM. The parallel test data is input to the parallel to serial converter where it is serialized in the normal manner in preparation for transmission over the serial link. However, rather than being output to the laser driver, the output from the parallel to serial converter is looped back to the serial to parallel converter on the receive side of the GLM. The serial to parallel converter deserializes the data and writes the data to the interface controller where it is checked against the original test data. During this operation the link is unavailable for transmitting real data. L.sub.-- UNUSE is short for link unusable. The L.sub.-- UNUSE signal is generated by the GLM and indicates when the link is available to transmit data. Among other conditions, this signal will be set true (logic 1) whenever the E.sub.-- WRAP signal initiates the loopback function, indicating that the transmit laser has been disabled and the link is unavailable.
GLM configured in the point-to-point configuration can support Open Fiber Control (OFC). In these modules the L.sub.-- UNUSE signal is also set true while the GLM establishes the integrity of the fiber optic link between two modules. This check is necessary to protect personnel against the damaging effects that high energy short wavelength laser radiation can have on human eyes when viewed directly. OFC ensures that the optical fibers extending between the transmit and receive ports of each GLM are in place and unbroken so that the laser radiation emitted from the laser transmitters is wholly contained within the fibers and therefore cannot be viewed directly by personnel working in the same area as the fiber optic link. OFC verifies the integrity of the optical fiber link by transmitting a low duty cycle test signal from one GLM to the other. If a fiber is broken or disconnected, the low duty cycle test signal escaping from the broken or disconnected fiber will have insufficient power to cause harm if inadvertently viewed by personnel in the area. If the fibers are intact, the second GLM receives the test signal and retransmits the signal back to the first GLM. Proper reception of the test signal at the first GLM indicates that the entire link is intact, and that both GLM may begin transmitting at full duty cycle. The L.sub.-- UNUSE signal is set true throughout the OFC test procedure.
OFC is not supported in all GLM, including those operating in the arbitrated loop configuration. The daisy-chain interconnection between GLM in an arbitrated loop does not allow a test signal to be transmitted from a first GLM then immediately retransmitted from a second GLM back to the first GLM. In the arbitrated loop configuration, the test signal would have to be transmitted all the way around the loop before being received back at the originating GLM. Without OFC, there is no way to determine the integrity of the fiber optic links between GLM comprising the loop. Because the arbitrated loop GLM cannot verify the integrity of the fiber links, the output power of these modules is limited to an output power level much lower than the output power level allowed for GLM supporting OFC. The output power of non-OFC GLM is held below the threshold which can harm the human eye. With the laser output power maintained below this threshold, the signals can be viewed directly without the threat of injury, eliminating the need for OFC. Eliminating the OFC check frees the L.sub.-- UNUSE signal from being held true (logic 1) while the integrity of the fibers is being checked.
A problem with the operation of arbitrated loop GLM has been the introduction of false control signals generated from random noise within the receiving circuitry of the GLM when one or more of the optical fibers has been disconnected. Even though the photodiode on the receiving side of the GLM may not be receiving optical signals over the fiber optic link, noise present in the highly sensitive photodiode and preamplifier circuits can generate random bit patterns which are transmitted to the host device through the interface controller. Occasionally the random bit pattern generated by the receiving circuitry will mimic valid Fibre Channel control signals. The interface controller will respond to these false signals according to predetermined rules established by the link protocol, such as for example, the Fibre Channel protocol. The interface controller will send a request to the host device to validate and accept the data. However, since the noise signal generating the request is invalid, the host will fail to validate the data based on higher level error checking functions, such as Cyclic Redundancy Checking (CRC). Requests for validation which are not validated by the host device are counted as elastic store errors by the interface controller. A Fibre Channel link is intended to be an error free data transmission medium. Thus, the interface controller attempts to resolve all data transmission errors. However, when the number of unresolved elastic store errors reaches a certain predetermined limit, the controller assumes there is a critical problem and initiates a higher level system shutdown. The system shutdown will alert the network manager, or some other service personnel that there is a problem with the arbitrated loop network, and that corrective action is required.
Obviously, it is undesirable to shut down the arbitrated loop network because of elastic store errors resulting from noise generated within the GLM, especially when the elastic store errors are generated during periods when the GLM is not actively receiving optical data signals over the transmission link. Therefore, it is desirable to provide a GLM with a received signal detect function which can generate a Signal Detected signal to alert the interface controller when data signals are being received over the fiber optic link. False data signals generated by noise during those periods when a data signal is not being received can be ignored. By ignoring these signals, the number of falsely accumulated elastic store errors can be reduced, as well as the amount of down time on the Fibre Channel network.
The GLM Family Specification does not provide for a signal detected signal to alert the interface controller when optical signals are actually being received over the fiber optic link. Therefore, there is no way to distinguish between valid data signals and noise generated in the GLM's receiving circuitry. Furthermore, the GLM Family Specification explicitly states that the L.sub.-- UNUSE signal should not be used for this purpose. The primary reason for this restriction is that L.sub.-- UNUSE must be held true (logic 1) during the entire OFC operation, regardless of whether optical power is actually being received over the fiber optic link. Since all of the interface signals between the interface controller and the GLM have been predefined by the GLM Family Specification, and since the L.sub.-- UNUSE signal has been unavailable for use as a signal detected signal, this feature has not been included in past GLM. The drawback from not including a received power detect signal in arbitrated loop GLM has been increased numbers of elastic store errors, and an unnecessary increase in the amount of network down time.